Semidetached Mass-transfer Massive Binaries in the Nearby Galaxy M31

Semidetached binaries provide a valuable test of the evolutionary models of interacting binaries where the mass is transferring from one component to the other, and the evolutions of both components will be changed. Many results of semidetached binaries have been obtained in the Milky Way, while there are few studies in the Andromeda galaxy (M31). By analyzing the light curves of 12 eclipsing binaries observed by Vilardell et al., we determined the fundamental parameters of these systems using the Wilson and Devinney method. It is discovered that they are semidetached binaries where the less massive components have filled their Roche lobes, while the more massive ones are detached from the lobes. We calculated their times of light minimum for the first time. The primary stars in those semidetached binaries have different mass ratios and fill-out factors and they are good sources to investigate the binary evolution in M31. It is found that the mass ratio is correlated with the fill-out factor of the primary. Systems with higher mass ratios tend to have lower fill-out factors. These results can be explained that the mass transfer from the less massive component to the more massive one. Moreover, the photometric solutions reveal that the secondary components are more evolved than the primary ones indicating that they are formed from originally detached binary systems via a case A mass transfer. These binaries have experienced the mass-reverse evolution and are at the late stage of a case A evolution in the nearby galaxy M31.